The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with magnetic resonance imaging of the spine in permanent C-magnet magnetic resonance imaging systems and will be described with particular reference thereto. However, it is to be appreciated that the present application will also find application in conjunction with other magnetic resonance imaging and spectroscopy systems in which the B.sub.0 primary magnetic field is orthogonal to the plane of the radio frequency coils.
Conventionally, magnetic resonance imaging procedures include disposing the patient in a substantially uniform, primary magnetic field B.sub.0. Magnetic resonance is excited in dipoles which preferentially align with the B.sub.0 field by transmitting radio frequency excitation signals into the examination region and receiving radio frequency magnetic resonance signals emanating from the resonating dipoles.
Most commonly, the B.sub.0 field is generated along the central bore of an annular magnet assembly, i.e., the B.sub.0 field aligns with the central axis of the patient. Cylindrical radio frequency and gradient magnetic field coils surround the bore. In order to improve the signal-to-noise ratio, quadrature surface coils have been utilized to examine a region of interest in quadrature, i.e., to receive signal components that are perpendicular to the coil and components that are parallel to the coil. See, for example, U.S. Pat. No. 4,918,388 of Mehdizadeh, which illustrates a loop coil and a flat Helmholtz coil, both of which receive resonance signals from the same region. The loop and flat Helmholtz coils are sensitive to orthogonal components of the magnetic resonance signal emanating from dipoles that are aligned with the horizontal magnetic field. When the output of one of the loop and flat Helmholtz coils is shifted by 90.degree. and the two signals are combined, the signal-to-noise ratio is improved by about .sqroot.2.
In order to examine larger regions of patients disposed in the bore of a horizontal B.sub.0 field imager, surface coils consisting of a plurality of loop coils have also been used. See, for example U.S. Pat. No. 4,825,162 of Roemer and Edelstein. More specifically, a series of loop coils are partially overlapped in order to examine contiguous regions. As explained mathematically by Grover in "Inductance Calculations" (1946) and summarized in the Roemer and Edelstein patent, the mutual inductance between adjacent coils is minimized when the coils are positioned by a slight overlap. Although the use of overlapped loop coils with the induction minimized enabled a larger area to be examined, each coil was linear. That is, each coil was sensitive to only one component and not sensitive to the orthogonal component such that no quadrature detection was provided.
U.S. Pat. No. 4,721,913 of Hyde, et al. illustrates another surface coil technique for horizontal field magnets. A series of linear coils are arranged continuous to each other, but with each coil disposed 90.degree. out-of-phase with adjacent coils. Thus, each coil received a radio frequency magnetic resonance signal component that was orthogonal to its neighbors.
In U.S. Pat. No. 5,394,087 of Molyneaux, a loop and flat Helmholtz coil are superimposed to provide a flat quadrature coil. A plurality of these flat quadrature coils are partially overlapped to define a planar, quadrature coil array.
While the above-referenced surface coils are effective for horizontal B.sub.0 field magnetic resonance imaging equipment, all magnetic resonance imaging equipment does not employ a horizontal B.sub.0 field. C-magnet magnetic resonance imagers include a pair of parallel disposed pole pieces which are interconnected by a C or U-shaped iron element. The iron element may be a permanent magnet or can be electrically stimulated by encircling coils to a magnetic condition. Typically, the pole pieces are positioned horizontally such that a vertical field is created in between. Thus, in an annular bore magnetic field imager, the B.sub.0 field extends between the patient's head and feet (or feet and head); whereas, in a C-shaped magnet the B.sub.0 magnetic field extends between a patient's back and front (or front and back). Due to the 90.degree. rotation of the B.sub.0 field, quadrature surface coils such as illustrated in the above-referenced U.S. Pat. No. 5,394,087, when positioned along the patient's spine in a vertical B.sub.0 field magnetic resonance imager, would not function in quadrature. They would loose sensitivity to one of their modes.
The present invention provides a new and improved radio frequency coil that provides quadrature reception/transmission in vertical B.sub.0 field magnets.